Structural State of plagioclases and petrogenesis

Abstract

This study pertains to the determination and analysis of the structural state of plagioclases and to evolve a petrogenetic model for the anorthosites of the Holenarasipur Supracrustal Belt, based on geological relations, petrography, mineral chemistry and geochemical criteria. Multivariate geostatistical methods have also been employed in the analysis of geochemical data. A brief summary of the findings is presented here. The Peninsular Gneiss Complex, the older Sargur Group, the younger Dharwar Supergroup and the younger granitoids are the major crustal components of the Archaean Karnataka Craton in the southern Indian shield. The older Sargur Group is preserved under the younger Dharwar Supergroup in the unique trident-shaped Holenarasipur Supracrustal Belt (HNSB), which is surrounded by the Peninsular Gneisses. A measurable success has been achieved in the analysis and synthesis of existing data to establish the relationship between these three components. The gabbro-anorthosite suite of rocks confined to the Sargur Group in this belt has yielded an emplacement age of 3095 Ma, while the amphibolites of the Dharwar Supergroup yield an age of 2600 Ma. The Peninsular Gneiss surrounding the HNSB has recorded an age of about 3071 Ma. The similar age relations and low Sr isotope ratios of anorthosites and gneisses suggest that they are contemporaneous and may be comagmatic. Petrographic studies of the Holenarasipur anorthosites reveal relict primary igneous texture and the predominance of C twins in plagioclases, characteristic of magmatic rocks. The amphibole compositions of HNSB anorthosites are predominantly Mg hornblendes extending into the tschermakite field, unlike those observed in the associated ultramafics, which are tremolitic or actinolitic. The mineral and whole rock chemical trends are clearly brought out in the APM diagram. The calc alkaline nature of the magma that gave rise to the anorthosite suite is indicated. The plagioclases of HNSB anorthosites range in composition from An to An . The structural state of plagioclases has been determined by X ray powder diffraction method. Unlike the low temperature plagioclases observed in layered Archaean anorthosites, high temperature plagioclases are predominant in the Holenarasipur anorthosites. It is interpreted that the disordered structural state of plagioclases is most probably due to elevated temperatures and water pressures attained during deformation and metamorphic recrystallization. Further, EPR spectral studies reveal that the high structural state is due to the occupancy of Fe³ ions in the distorted tetrahedral sites in the plagioclase structure. The absence of Ti spectral component in the plagioclases indicates that the plagioclase crystals were formed under oxidising environment unlike those of lunar plagioclases. Field relations imply that the anorthosite suite has genetic relations with the gneisses and the ultramafics in HNSB. However, mineral chemistry rules out the genetic relationship between the anorthosites and the ultramafics. In the CaO-(FeO + Fe O + MnO) ternary variation diagram, the anorthosite suite displays a vertical trend whereas the ultramafics define an oblique trend. There is an obvious gap between the two trends, suggesting that the ultramafics and the anorthosite suite are not genetically related. The low initial Sr isotope compositions of the anorthosite suite (0.7016) and the gneisses in the vicinity (0.7008-0.7020) indicate the comagmatic character of the two. The whole rock Rb/Sr ages of both suites are also almost similar (about 3100 Ma). An examination of chondrite normalized REE abundance levels shows that the anorthosites with low abundances exhibit positive Eu anomaly, while the gneisses with high abundance levels show negative Eu anomaly. Thus, the REE data highlight the profound geochemical and petrogenetic differences between the anorthosites and the gneisses. Assuming basaltic andesite to be the most likely parent magma, it can be interpreted that the anorthosite suite formed an early cumulate sequence and the silicic gneisses formed out of the residual liquid. Computer analogues of graphical tests (R mode cluster analysis and Q and S mode factor analyses) have also been presented to show the genetic relation of anorthosites with the gneisses.